In my opinion, the best route is to go east from Seattle through Boise, Idaho, Salt Lake City, then south to Phoenix. The roads through Idaho and Utah are uncrowded and high quality, and the scenery is fantastic. Cross the border at Nogales and follow Highway 200 through Mexico. The coastal route through Mexico is 4-lane toll roads to Tepic. The tolls are expensive (about $100) but worth the ease of driving.